Add Developer CLI command to easily amend commits with co-authors (#683)

### Summary & Motivation

Introduce a new `amend-as-coauthor` CLI command to easily annotate a Git
commit with a co-author. This simplifies the process of making small
adjustments to commits while properly crediting maintainers. The command
appends a `Co-authored-by:` line to the commit message, ensuring clean
and traceable contributions in PlatformPlatform.

### Checklist

- [x] I have added tests, or done manual regression tests
- [x] I have updated the documentation, if necessary

developer-cli/Commands/AmendAsCoAuthorCommand.cs

@@ -0,0 +1,63 @@
+using System.CommandLine;
+using System.CommandLine.NamingConventionBinder;
+using PlatformPlatform.DeveloperCli.Installation;
+using PlatformPlatform.DeveloperCli.Utilities;
+using Spectre.Console;
+
+namespace PlatformPlatform.DeveloperCli.Commands;
+
+public sealed class AmendAsCoAuthorCommand : Command
+{
+    private const string CoAuthorTrailer = "Co-authored-by";
+
+    public AmendAsCoAuthorCommand() : base("amend-as-coauthor", "Amends the current commit and adds you as a co-author")
+    {
+        Handler = CommandHandler.Create(Execute);
+    }
+
+    private static int Execute()
+    {
+        var gitUserName = ProcessHelper.StartProcess("git config user.name", Configuration.SourceCodeFolder, true).Trim();
+        var gitUserEmail = ProcessHelper.StartProcess("git config user.email", Configuration.SourceCodeFolder, true).Trim();
+
+        if (string.IsNullOrEmpty(gitUserName) || string.IsNullOrEmpty(gitUserEmail))
+        {
+            AnsiConsole.MarkupLine("[red]Git user name or email not configured.[/]");
+            Environment.Exit(1);
+        }
+
+        var commitAuthor = ProcessHelper.StartProcess(
+            "git log -1 --format=\"%an <%ae>\"", Configuration.SourceCodeFolder, true, throwOnError: true
+        ).Trim();
+        var currentUser = $"{gitUserName} <{gitUserEmail}>";
+        if (commitAuthor == currentUser)
+        {
+            AnsiConsole.MarkupLine("[yellow]You are already the author of this commit.[/]");
+            Environment.Exit(0);
+        }
+
+        var stagedChanges = ProcessHelper.StartProcess("git diff --cached --name-only", Configuration.SourceCodeFolder, true);
+        var hasNoChanges = string.IsNullOrWhiteSpace(stagedChanges);
+
+        var commitMessage = ProcessHelper.StartProcess("git log -1 --format=%B", Configuration.SourceCodeFolder, true).Trim();
+        var coAuthorLine = $"{CoAuthorTrailer}: {currentUser}";
+        var isAlreadyCoAuthor = commitMessage.Contains(coAuthorLine);
+
+        if (hasNoChanges && isAlreadyCoAuthor)
+        {
+            AnsiConsole.MarkupLine("[yellow]No staged changes, and you are already a co-author of this commit.[/]");
+            Environment.Exit(0);
+        }
+
+        if (hasNoChanges && !AnsiConsole.Confirm("No staged changes. Do you want to add co-author information only?"))
+        {
+            Environment.Exit(0);
+        }
+
+        var amendMessage = isAlreadyCoAuthor ? "--no-edit" : $"-m \"{commitMessage.TrimEnd()}\n\n{coAuthorLine}\"";
+        ProcessHelper.StartProcess($"git commit --amend {amendMessage}", Configuration.SourceCodeFolder);
+        AnsiConsole.MarkupLine("[green]Successfully amended commit with co-author information.[/]");
+
+        return 0;
+    }
+}